The Senate stood poised Thursday to approve historic immigration legislation opening the door to U.S. citizenship to millions and promising a dramatic build-up of manpower and technology along the U.S.-Mexico border. The vote on final passage of the White House-backed bill was expected as early as Thursday afternoon, after a series of test votes so far this week demonstrated supporters command a bipartisan majority well over the 60 votes needed to secure passage and send the bill to the House. First must come two more procedural tests set for Thursday morning. …